1. Field of the Invention
The present invention relates to hydrocarbon separation, and more particularly to liquid-liquid solvent extraction, vapor-liquid solvent extraction (also known as extractive distillation) and a system adapted for the practice thereof.
2. Description of the Related Art
Aromatic hydrocarbons, such as benzene, toluene and xylenes (collectively, “BTX”), serve as important building blocks for a variety of plastics, foams and fibers. Often these compounds are produced via catalytic reformation of naphtha through steam cracking of naphtha or gas oils, or other methods where substantial amounts of non-aromatic compounds are present. When simple distillation or fractionation is not a cost effective or practical method for separation, liquid-liquid extraction or vapor-liquid extraction techniques are used. Such extraction techniques separate a desired substance selectively from a mixture or remove unwanted impurities from solution, and, in the present context of aromatic hydrocarbon separation from non-aromatic hydrocarbons, typically use a solvent which exhibits a higher affinity for the aromatic compounds than the non-aromatic compounds, thereby selectively extracting the aromatic compounds from the mixture of aromatics and non-aromatics. The aromatic species of interest can then be isolated from the solvent by distillation, adsorptive separation techniques, and the like.
One widely used solvent extraction technique is the Sulfolane™ process developed by UOP, which is discussed in numerous patents and other literature too numerous to cite. The process uses a combination of liquid-liquid extraction and extractive distillation in a single, integrated design, and employs tetrahydrothiophene-1,1-dioxide (or sulfolane) as a solvent and water as a co-solvent. One of the problems with this process is that light impurities have a tendency to buildup in the system, such as in one or more distillation towers and recycle streams. Without wishing to be bound by theory, this may be due at least in part to sieve tray fouling or high unit rates, both reducing disengaging times. These undesired effects result in the incapacity of the extractor to efficiently remove and recover the aromatic compounds within the mixed feedstock and may also lead to leaks due to corrosion, the latter of which may have catastrophic results.
Other solvents conventionally used in liquid-liquid extraction include oxygen-containing species such as tetraethylene glycol and nitrogen-containing species such as N-methylpyrrolidine, each having similar issues, as well as their own unique problems, that lead to decreasing the capacity of the extraction process.
Typical responses to correcting the incapacity of the extractor include one or more of moving the recycle location, adding more stages of sieved trays, reducing operating rates, or cleaning and/or replacing the sieve tray decks. Numerous other solutions have been disclosed, such as in U.S. Pat. No. 7,288,184, and U.S. Publication 2010-0096321. Other references of interest include U.S. Pat. Nos. 7,288,184; 3,720,605; and 2,878,182; and FR 2079236.
The present inventors have discovered that proper control of the solvent systems in the separation of aromatic hydrocarbons from non-aromatic hydrocarbons, including liquid-liquid extraction processes and extractive distillation processes, and the combination thereof, can result, in embodiments, in at least an order of magnitude improvement in reliability and integrity of the solvent systems and processes for the separation of aromatic hydrocarbons and non-aromatic hydrocarbons.